Supercritical fluids such as carbon dioxide are becoming a popular alternative to organic solvents for extracting chemical species of interest from a host medium. For example, the extraction may be employed in the chemical analysis of liquid or solid samples or in a separation process in the chemical processing industry. A supercritical fluid is contacted with the host medium and the species of interest are solubilised by and extracted into the fluid. After the extraction has taken place, the extracted species are removed from the supercritical fluid by depressurising the fluid to a gas to enable the extracted species to be precipitated in a collection device which may for example contain an organic collection solvent to facilitate collection. In order to maintain the extraction pressure in the system and simultaneously to depressurise the supercritical fluid in a controlled manner, a flow restrictor acting as a back pressure regulator is required. However, since the depressurisation occurs inside the flow restrictor or at the flow restrictor output tip, the reduction of the fluid density, combined with the Joule Thomson cooling effect which occurs at the restrictor tip, can cause a decrease in the solubility of the extracted species which leads to precipitation and ultimately plugging of the restrictor.
Restrictor plugging is a very common occurrence during a supercritical fluid extraction particularly when the fluid is saturated with extracted species. One approach to preventing restrictor plugging is to use a mechanical and/or electrical feedback regulator with a variable orifice such as that used by various manufacturers, for example, Hewlett-Packard (European Patent EP 384969A2), Suprex (Technical note PD--12,9/92) and JASCO Corporation (M. Saito, T. Hondo and Y. Yamauchi in "Supercritical Fluid Chromatography", RSC Chromatography Monographs, London, 1989, page 203).
European Patent Application No. EP 384969A2 discloses a variable orifice which allows flow rates to be set independently of density and temperature. The size of the orifice at the restrictor nozzle varies depending on the pressure required. The extracted solutes are deposited in a sorbent/collection trap which after the extraction is flushed with organic solvent to elute the solutes into a collection vial.
The Suprex Corporation discloses in the reference noted above a variable automated restrictor which has an electronic sensor that registers restrictor plug formation and automatically opens and closes the restrictor to provide a uniform flow. The extracted species are collected in a collection module which consists of a collection trap that retains the species during the extraction, and a solvent pump that pumps solvent through the trap (after the extraction) to transfer the solutes into a collection vial.
The JASCO Corporation disclose in the reference noted above a variable flow restrictor which has a pressure sensor with a feedback mechanism which controls the opening and closing of the exit orifice depending on whether the extraction pressure is above or below the required value. The solutes released from the back pressure regulator are collected at atmospheric conditions inside a collection device.
An alternative approach to the mechanical and/or electrical feedback regulator is a linear flow restrictor constructed from tubing with a fixed internal diameter. To avoid restrictor plugging the linear restrictor is heated, as heating the restrictor counteracts the Joule-Thomson cooling effect at the restrictor tip and increases the supercritical fluid solubility of species having some volatility. Several manufacturers use a heated linear flow restrictor, namely: Suprex, U.S. Pat. No. 5,205,987, Dionex, International Patent Application No. WO92/06058, and ISCO, U.S. Pat. No. 5,268,103.
U.S. Pat. No. 5,203,987 discloses a restrictor which is constructed as a precisely machined stainless steel orifice which converts the solutes from a high pressure to a low pressure environment. The low pressure side is in fluidic communication with the collection means.
International Patent Application No. WO92/06058 discloses a temperature controlled fused silica restrictor secured inside a stainless steel tube by means of an epoxy resin. Most of the restrictor is directly heated and the extracted species are collected in an organic solvent.
U.S. Pat. No. 5,268,103 discloses a temperature controlled stainless steel restrictor that is completely heated. The end of the restrictor is placed in an organic solvent so that the solutes can be collected directly into a liquid collection solvent.
All of these known systems are very expansive and several are not amenable to direct collection of extracted species in an organic solvent. Furthermore, they all rely on heat or the added complication of an orifice opening and closing device to avoid restrictor plugging.
The purpose of the present invention is to provide a new approach to the aforementioned problem of restrictor plugging.